The presence of nebular Heii emission points to the existence
of a hard radiation field as photon energies \(\ge \)  54 eV are necessary
to ionize He\(^{+}\) . Highly ionized systems, like Heii emitters,
are expected to be more frequent at high-z
[1]}, [2]}. At the same time, observations
of local, Heii-emitting star-forming (SF) galaxies suggest that the lower the metallicity (Z), the larger the nebular
Heii line intensities [3]}, [4]}. This is in
line with the harder ionizing stellar spectra predicted at the lower
metallicities typical in the early universe [5]};
the first, hot metal-free Population III stars produce
strong nebular Heii lines
[6]}, [7]}. Still, despite much
observational and theoretical effort, including relevant advances in
stellar modeling, the source of Heii ionization remains puzzling, especially in metal-poor galaxies
[8]}, [9]}, [10]}, [11]}, [12]}.
